EXPT 3. Elemental Analysis and
Freezing Point Depression
[Key Contents]
- empirical formula, molecular weight, molecular formula, colligative
properties
- elemental analysis of glucose and sucrose, gas chromatography
- determination of molecular weight from freezing point depression
- development of chemistry in the 19th century
[References]
Principles of Modern Chemistry, 6th Ed. (Oxtoby et al.)
Ch. 2. Chemical Formulas, Chemical Equations, and Reaction
Yields
Ch. 11. Solutions
Chemistry for Life, Chemistry for Better Life (Kim et al.)
Ch. 1. Introduction
Ch. 3. Atoms and Molecules
Ch 7. States and Properties of Matter
Ch 9. Equilibrium Reactions
[Goal]
Once a new material is either discovered or synthesized, one needs to
determine
its
chemical
structure,
because
the
properties
and
functionalities of the material depends on the chemical structure. To
determine chemical structure, one needs to know the molecular weight.
Using glucose, ribose, and sucrose, you will learn how to get empirical
formula by elemental analysisand then molecular formula by freezing
point depression experiment.
[Background]
Supposew that you isolated glucose for the first time and learned that
it consists of carbon, hydrogen, and oxygen only. To determine its
molecular weight and molecular structure, you need to determine
weigh percent of carbon, hydrogen, and oxygen by elemental analysis.
Carbon
and
hydrogen
are
found
in
all
organic
compounds;
therefore, their determination was an important issue in the early
days of organic chemistry. In 1831 Liebig in Germany established
analytical procedures for carbon and hydrogen. The sample was
burned and the carbon dioxide resulting from carbon was captured in
an alkaline solution and weighed. Water from hydrogen was captured
by absorbents such as P2O5 and weighed. Oxygen was determined by
difference.
Elemental analysis only yields the empirical formula. One needs to
know the molecular weight to determine the molecular formula and the
molecular structure. In the 19th century when mass spectrometry was
not available, molecular weight could be estimated from colligative
properties of solution (freezing point depression, for example).
Preparatory Problem : Draw the chemical structure of glucose,
ribose,
and
sucrose.
Calculate
elemental
composition
of
these
compounds.
[Apparatus and Chemiclas]
4 glass vials, distilled water, 250 ml beaker, ice, 2 temperature
sensors, stop watch, stand
elemental analyzer
Sample 1, 2 (glucose, ribose, or sucrose),
elemental analyzer
freezing point appratus
[Procedure]
Expt 1. Elemental Analysis and Empirical Formula
1) The lab assistant will demonstrate the use of the elemental
analyzer. Several groups will share the results obtained from unknown
samples (glucose, ribose, sucrose).
2) Place a tin-foil weighing boat on the precision electronic balance
and set zero.
3) Take 1~2 mg sample, tightly fold the weighing boat, and accurately
weigh.
4) Load the sample on the auto-sampler.
5) Enter the sample weight and start analysis.
Expt 2. Freezing Point Depression and Molecular Formula
1) Weigh about 1 g each of the given samples to 0.01 g accuracy,
transfer to separate vials, and dissolve in 5.0 mL distilled water.
2) Mixing ice water with enough salt in a 250 ml beaker, prepare
ice/water bath below -10oC. Add ice and salt as necessary. Occasionally
check the temperature of the ice/salt mixture.
3) Immerse one of the vials with the sample solution into the ice/water
bath. Stir the sample solution with the temperature sensor. When the
temperature drops to about 2oC, take the temperature reading every 10
seconds for about 10 minutes and watch the change in temperature.
Initially, temperature of the sample solution will drop below the freezing
point by super-cooling and come back up to the freezing point once
freezing starts. A constant temperature will be maintained while freezing
before the temperature drops further once the entire solution is frozen.
4) Repeat using the other sample solution.
5) Repeat using distilled water as a control experiment.
6) If time permits, repeat using a solution of NaCl. Use a
concentration that will depress the freezing point as much as one of
the samples.
[Data Analysis]
1. Use the results from the elemental analysis to obtain the empirical
formula for the unknown samples.
2. Plot temperature against time for the sample solutions. Use the data
from distilled water as a control. Determine molecular weight of the
samples and identify them as glucose, ribose or sucrose.
Expt 1
calculated weight %
glucose
ribose
sucrose
observed weight %
sample 1
sample 2
-------------------------------------------------------C
H
O
empirical formula of sample 1 :
empirical formula of sample 2 :
tentative match : sample 1 - (glucose, ribose, sucrose)
sample 2 - (glucose, ribose, sucrose)
Expt 2
Sample Data
freezing point of distilled water : (
freezing point of sample 1 : (
) oC
) oC
freezing point depression of sample 1 : (
M.W. of sample 1 : (
)
freezing point of distilled water : (
freezing point of sample 2 : (
) oC
) oC
freezing point depression of sample 2 : (
M.W. of sample 2 : (
) oC
) oC
)
Conclusion : Sample 1 is (glucose, ribose, sucrose).
Sample 2 is (glucose, ribose, sucrose).
Elemental analysis played a key role when Avery showed that DNA is the
genetic marerial.
[Additional Material]
1. Elemental Analysis of DNA
Oswald Avery
2. Analytical Configuration of the Elemental Analyzer
3. Typical Gas Chromatogram of the Elemental Analyzer
4. Liebig Who Developed the Elemental Analysis
Justus von Liebig (1803-1873)